Ion source



W. A. S. LAMB Aug. 25, 1959 ION SOUR'CE AMPLIFIER Filed Dec. 31, 1954INVENTOR.

WILLIAM A. S. LAMB ATTORNEY.

ION SOURCE William A. S. Lamb, Berkeley, Calif., assignor, by mesneassignments, to the United States of America as represented by theUnited States Atomic Energy Commission Application December 31, 1954,Serial No. 479,255

4 Claims. (Cl. 250-495) The present invention relates to an improved ionsource wherein the ion extracting fields do not interfere with thefields employed in the mechanism of ion production and a simplified andreadily controllable pulsed ion extraction is provided.

One of the greatest difficulties in the production of an ion beam foruse in apparatus, such as particle accelerators for example, is theextraction of ions from the ion generating means. One difficulty lies inthe extraction of a sufiicient number of ions from the generator, eventhough an adequate number are generated. Additional difficulties lie inthe formation of an ion beam that can maintain a very high intensitywithout blowing up and in controlling ion extraction to produce an ionbeam having desired current variations.

The ion source of the present invention overcomes the above-noteddifiiculties. The source configuration and ion extraction pattern issuch that a hollow ion beam is produced wherein space charge effectsnormally limiting beam intensity are materially reduced. Ion extractionis accomplished by the establishment of fields which do not counteractor disturb the ion generating fields so that ion generation issubstantially independent of extraction and suffers no interferencetherefrom. Also, the ion extraction means operates upon a transformerprinciple with high frequency feedback control, whereby ready andaccurate extraction control provides desired ion beam currentvariations.

It is an object of the present invention to provide an improved ionsource having a high frequency controlled magnetic field ion extractionmeans providing phased ion extraction.

It is another object of the present invention to provide an iongenerator having a radially extending ion sheath with means establishingan ion extracting magnetic field acting thereon and controlled by highfrequency feedback signals from ion beam utilization apparatusassociated with the source.

It is a further object of the present invention to provide, inconjunction with an ion source and an accelerator, an ion extractingmechanism including magnetic field producing means for ion extractionand controlled by accelerator voltages.

Numerous other advantages and possible objects of the invention willbecome apparent to those skilled in the art from the followingdescription of a single preferred embodiment of the invention takentogether with the accompanying drawing wherein:

Figure l is a central sectional view of the ion source in schematicrepresentation with relative potentials of elements indicated byconnection to batteries; and

Figure 2 is a partial sectional view taken at 22 of Figure 1.

Considering now the details of the illustrated embodiment of the presentinvention and referring to the drawings, there will be seen to beprovided magnet structure including a toroidal pole piece 11 and acentral cylindrical pole piece 12 with same being coaxially disposed toice define a toroidal space therebetween. These pole pieces comprise theends of magnet cores adapted for energization as by electrical windingsto establish a magnetic field therebetween with the central pole piececonsisting, for example, a south magnetic pole and the surrounding polepiece 11 a north magnetic pole, so that a radial magnetic field isestablished through the toroidal space separating the pole pieces.Within this space there is disposed an annular conductor 13 about thepole piece 12 and adapted to emit electrons upon electricalenergization. Extending radially outward from the vicinity of thecentral pole piece 12 are electrode means 14 which preferably in cludesan annular back wall 16 and a parallel annular front wall 17 which maybe mounted in any suitable manner in insulated relation to the emittingconductor 13 and between the pole pieces. This electrode means comprisesan anode 14 in that it is adapted to attract electrons.

With the aforesaid disposition of elements, electrons emitted from theemitting conductor or filament 13 are attracted by the anode structure14, but are constrained from reaching same by the radial magnetic fieldbetween the pole pieces 11 and 12 in a well-known manner and the emittedelectrons thus travel radially outward toward the pole piece 11. Thedischarge is intensified by the provision of a pair of reflectorscomprising an outer annular metal strip reflector 18 disposed directlyinward of the outer pole piece 11 and a second annular metal stripreflector 19 disposed between the inner pole piece 12 and the filament13. These reflectors 18 and 19 extend longitudinally across the polepieces about the inner and outer circumferences thereof respectively andare adapted to be maintained at a relatively negative potential wherebyelectrons are repelled therefrom. The anode 14 and reflecting electrodes18 and 19 define a toroidal chamber 20 within which the dischargeoccurs. The electron discharge becomes oscillatory in that electrons arereflected back and forth between the two reflectors 18 and 19 within thechamber 20 and an elongated electron path is thereby produced.

Ionization is accomplished by the introduction of gas or vapor into theoscillatory electron discharge with the gas molecules being ionized bycollision with electrons in the discharge. The gas or vapor to beionized may be admitted to the discharge space by means of a slot orslots 21 through the back plate 16 of the anode 14 and about same withan annular pipe 22 communicating with the slot for carrying the gas orvapor to be ionized. An ion egress aperture 23 is provided in the formof a slot about the front plate 17 of the anode 14 with this slot, orslots, extending circumferentially thereabout near the radial centerthereof.

It is to be appreciated that the embodiment of the invention hereinshown and described is merely illustrative and that numerous structuralmodifications and variations are possible. With regard to the mountingof elements of the ion generator within the space between the magnetpole pieces for example, a hollow insulating toroid may be formed ofglass or the like with suitable exterior mounting and carrying theanode, filament and reflecting electrodes in suitable spaced relationtherein, or these elements may be separately mounted upon insulatorswhich are in turn secured to exterior mounting structure. Likewise as tothe energization of the above-described members, there is to be providedenergizing windings for the magnet with a suitable power supplyconnected thereto for supplying magnet winding current and one or morepower supplies are provided for establishing and maintaining therelative potentiais of the ion generator elements and the filamentcurrent. There is shown as representative of these power supplies afirst battery 24 having a grounded negative terminal and connectionsfrom thepositive terminal to the front and back walls of the anode 14. Asecond battery 25 has a grounded positive terminal and connections fromthe negative terminal thereof to the filament 13and reflectingelectrodes 18 and 19.- A'-filament current supply is shown as a-thirdbattery 26 connected across leads 27" secured to ends of the filament13which are spaced apart to provide a'heating current path so thatcurrent'therethrough' renders thefilament electron emissive.

Considering now the extraction of ions from the abovedescribed iongenerator, there will be seen to be provided a pair of'concentrictoroidal magnet cores 31' and 32 di'sposedcoaxially with the iongenerator and lying in a plane parallel to and spaced from the planeofthe front wall 17 of'the anode 14. The annular space between the twotoroidal magnet cores 31 and 32, which may be termed an ion acceleratingspace or slit 33, is aligned with the opening 23 in the front wall 17'of the anode structure 14 so thata straight line of ion egress isprovided about a median circumference of the ionizing discharge. Theextracting -magnets include, in addition to the cores 31 and 32,energizing means including a loop 34 of one or more turns of electricalconductor about the cores, as shown. This energizing loop is adapted toconduct high frequency electrical currents to produce with the cores 31and 32' a transformer action; Electrical energization of the loop 34 is'derived from-an ion accelerator 36 with which the ion source is adaptedto cooperate and this accelerator may comprise a plurality of resonantcavities in which case the ion source is adapted to inject highfrequency bursts of ions into the cavity resonator of the particleaccelerator for acceleration ofthe ions therein, there being shown partof a wall 37 of this accelerator 36. A pickup loop 38 is disposed withinthe first cavity of the accelerator 36 for receiving electricalenergization in synchronism with field fluctuations in the cavity and alead connected to the pickup loop extends through the cavity wall37 ininsulated relation thereto and is connected to theinput terminal of aphase control circuit 39. The output of this phase control circuit 39.isconnected to the magnet loop 34 through an amplifier 41. The pickup loop38 receives energy from the cavity of the accelerator in phase with thecavity energization in a known manner and applies this energy inamplified form to the magnet loop 34 forestablishing a magnetic fieldabout the cores 31 and 32. The phase control circuit 39 operates toprovide small'phase corrections as required so that the magnetic fieldextracting ions from the ion generator and injecting same into theaccelerator shall have the proper phase relation to inject the bursts ofions at the desired accelerating field intensity within the particleaccelerator 36.

Considering now in rsum the overall operation of the ion source of thepresent invention, energization of the filament and generator electrodeswiththe establishmentof. a radial magnetic field between'the pole piecesHand 12 produces an electron discharge from the filament 13radiallyoutward. This discharge .is reflected by the electrode 18 radiallyinward and, thence, reversed in direction by the inner electrode 19 soas. to oscillate radially under the influence of the'magnetic fieldlimitingtransverse electron travel. Introduction of a gas or vaporintothe discharge area from the tube 22 produces ionization by bombardment,whereupon the ions so formed are constrained to the general area of thedischarge within the chamber 20. by the magnetic field between the polepieces 11 and 12 to form a generally planar toroidal ion sheath. Pulsedenergization of the first cavity of the particle accelerator 36, withwhich the ion. source is adapted to cooperate, energizes. the pickuploop-38 which in turnenergizes the magnet loop 34. Intermittent currentflow through the magnet loop 34-sets up a magnetic field in the toroidalcores 31 and 32. This, field alternately builds up and collapses withmagnetic lines of force extending through the ion exit slot 33--betweenthe cores 31 and 32'andinto the chamber 20 of the ion source. Thismagnetic field applies a force upon the ions and electrons in thechamber 20 causing them to precess about the annular discharge space,but does not otherwise deflect them except to provide an attractiveforce thereon normal to the plane of the discharge toward the-.ion exitslit 33 by virtue of the transformer action of charged particles cuttingmagnetic linesiof force..- Ofcourse, thegcuttingtresultsfromthesuccessive-riseand fall of' the-magnet-ic field'and, although noconductor lies between the cores 319 and 32, the free ions are actedupon much as if they lie alonga solid conductor between; the. cores.Consequently, ions are attracted from the toroidal ion sheath throughthe circular slit or egress aperture 23 and, thence, through the magnetcore separation or slit 33 whereby there emerges a hollow cylindricalion beam. A feature of-this emergent ion beam is the absence of limitingspace charge eflects for the beam is concentrated in a thinshellso thatinsulficient beam. cross section resultsfor large scale interaction ofion charges. The ion beam in hollow cylindrical form contains a largernumber of ions per unit length, i.e., beam current, than is possiblewith conventional beams. Further, with the improved ion extractionstructure, no interruption or interference with the mechanism of iongeneration results during recurrent periods of ion extraction and aready phase identity or relationship is established between ioninjection and accelerating forces.

It is noted that the ion source of the present invention including bothion generator and ion extractor may be readily employed with a varietyof apparatus utilizingion beams, and it is notnecessary that resonantcavities be employed in the particle. accelerator for the magnet loopmay be energized in conventional manner from any source of ionaccelerating potential employed in the accelerator.

What is claimed is:

1. An, improved ion source-comprising means establishing an annularionizing discharge, means providing gaseous material to said dischargewhereby ions thereof are produced, a ring of material having a highmagnetic permeability disposed adjacent said discharge coaxiallytherewith in a plane parallel thereto, and a loop about said ring forenergization with high frequency electrical energy for attracting ionsfrom said generator to form a hollow cylindrical ion beam.

2. An ion source comprising means establishing a radially oscillatingtoroidal electric discharge, means feeding vaporized source materialinto said discharge for ionizing same, a pair of coaxial toroidal.magnet cores having a high permeability disposed in a plane adjacentsaid discharge with the core separation aligned with at least a part ofsaid discharge, and a conductor looped about said cores andconnected forenergizationbyapparatus employing ions fromisaid source whereby saidcores establish an ion-extracting field removing'ions from the dischargeareain the form of a hollow cylindrical ion beam.

3. An ion source comprising a toroidalfilament, an anode attractingelectrons radially outward from said filament to establish a discharge,means introducing a material to be ionized'into said discharge forproducing a toroidal sheath of ions, a pair of toroidal magnet coresdisposed coaxially with said filament and displaced axially therefrom, aconductor looped about said cores, and means energizing said loop withhigh frequency energy forv establishing a magnetic field intersectingsaid ion sheath whereby ions generated in said discharge are extractedtherefrom.

4. An ion source for a cavity accelerator comprising means establishinga toroidal electric discharge, means feeding a vapor to be ionized intosaid discharge whereby ions. are formed therein by bombardment, a pairof co- 6 circuit and wound about said magnet cores for energizing sameto establish a magnetic field thereabout whereby ions are attracted fromsaid discharge in a hollow cylindrical ion beam.

No references cited.

